The field of application of this invention is in a warehousing or merchandising operation wherein information-bearing electronically coded labels are attached to objects which are to be identified, sorted, controlled or audited, by means of a process in which information passes between an interrogator, which creates an electromagnetic interrogation field, and electronically coded labels which respond by issuing a reply signal which is detected by the interrogator, decoded and supplied to other apparatus used in the sorting, controlling or auditing process. Variants of such systems are known, for example from Australian Patent 658857 of Marsh and Lenarcik or Australian Patent 664544 of Turner and Cole. In some variants of the system the interrogation medium may be other than electromagnetic, such as optic or acoustic.
In normal operation the labels may be passive (ie have no internal energy source and obtain the energy for their reply from the interrogation field), or active (ie contain an internal energy source, for example a battery), and respond only when they are within or have recently passed through the interrogation field which may have the function of signalling to an active label when to commence a reply or series of replies, or may in the case of a passive label provide energy a portion of which is used in constructing the reply.
A feature which is common to almost all systems is that it is desirable that the circuits within the labels be kept simple, so that they may be economically manufactured.
A common problem in such systems is that an unknown plurality of labels may be simultaneously present in an interrogation field, and the process of communication between interrogator and label must be structured so that all labels present in the interrogation field are detected.
One approach to this problem is presented in Australian Patent 664544 of Turner and Cole and is to design labels so that they reply intermittently and at unrelated intervals, without reference to timing signals issued by the interrogator, so that over time there is an increasing probability that a given reply will not have permanently overlapped with others, and will be correctly read and decoded by the interrogator.
A shortcoming of this system is that when labels with an inter-reply interval having only small variation are used, the system does not perform well when the number of labels simultaneously present in the interrogation field exceeds a number, roughly equal to the ratio of the average interval between replies from a single label to the time taken for a single label reply, such that there is a reasonable probability that after a label has replied a few times at least one of its replies will have occurred in a time interval not occupied by other label replies. When the number of labels simultaneously present in the interrogation field considerably exceeds this number, there is only a small chance that a given reply will not be interfered with by others, and a long time elapses before all labels have been successfully read.
A further shortcoming of this system is that it tacitly assumes that all labels present at one time in the interrogation field are differently coded, so that the interrogator has a means of distinguishing replies from all labels, and can give an indication of how many labels are in the field as well as giving their information content. In the case where it is desired to code the labels into classes, and have for labels which are to be attached to identical objects the same information content, it is no longer possible to distinguish the total number of labels in the field, or the total number of labels of each class in the field.
Another disadvantage is that labels once read, continue to interfere with labels still to be read, and the total time for reading all labels is thereby increased. It is also a disadvantage that there is no absolute guarantee that within any given time all labels will be read, there is still a statistical probability, albeit small, that some labels will have suffered interference throughout the period allocated to their interrogation.
One approach to this problem has been proposed in the Australian patent 658857 of Marsh and Lenarcik. In this solution the replies from labels are, as they are received, evaluated for their data content and the correctness of the data content, as evidenced by a series of check bits embedded into the reply, and as soon as a label reply has been correctly received, the correctness of the decoding is examined, and if it is found that the decoding was correct, and acknowledgment of correct reception is made by providing a brief interruption to the signal transmitted by the interrogator, and any label which has just replied is permanently turned off, ie it issues no further replies. It has been assumed in this disclosure that in this way each label of a number of identically coded labels can be individually detected, counted, and silenced, so that the total number of such identically coded labels present in the field can be determined.
There are several problems in this approach. One of these derives from the so-called small signal suppression effect, which is common with the phase modulated reply signals generally employed in label replies, and which has the result that if two labels reply at one time with significantly different reply signal strength, the stronger signal will be correctly decoded by the interrogator while the weaker signal will be ignored, with the result that both of the labels will cease transmission in response to the acknowledgment signal, although only one of them will have been counted. This problem will exist whether the labels are differently coded or identically coded.
Although in the case of differently coded labels the problem may in principle be dealt with by having the acknowledgment signal contain the entire label reply code, and arranging through a considerable increase in complexity of the label circuit so that it will only cease replying when it has received a specific acknowledgment which is clearly directed solely at itself, this solution is undesirable as it can only be implemented at the cost of considerable increase in complexity of the label circuit. More serious, however, is the fact that when a number of identically coded labels are simultaneously present, this remedy is completely ineffective, as there is no mechanism to prevent the occurrence of simultaneous replies from two identically coded labels with widely different reply strengths, as can easily occur as a result of significant variation of the interrogation signal strength throughout the scanned region, and independent significant variation in the orientation sensitive coupling between a label antenna and the interrogation field, with consequence that the pair of replies is seen as one.
It is an object of the present invention to at least alleviate the disadvantages and problems discussed above.
It is desirable for economy of manufacture to maintain a simple label design, and as an aspect of this to allow labels in their reply generation process to be substantially self-timed.
It is desirable to cater efficiently for a significantly varying plurality to labels in the interrogation field at various times, and to allow both for situations where labels all have different information content and for situations where groups of labels are coded with the same information content. In all of these situations it is desirable to provide a precise report of the identities all labels in field and in particular the number in each group of labels with same information content.
It is desirable to allow significant time for reply signal decoding and error checking in the decoding process.
It is also desirable to resist errors arising from the unpredictability of reply positioning and signal strength whereby a reply from one label may obscure a reply from another, and a weakly replying label may have its reply sequence mistakenly curtailed through reception of a signal intended for a strongly replying label.
Most of the above desirable features may be satisfied by allowing two way communication between the interrogator and the labels. In contrast to simpler systems in which the interrogator merely provides an energising signal for the label or a trigger signal telling the labels when to begin a sequence of replies, in this disclosure the interrogator, in addition to receiving and decoding the replies from labels, can send to the labels a limited number of information bearing signals, and the labels can act upon those signal in simple but useful ways.
The signals issued by the interrogator can be divided into classes. One class of signals may be issued at any time. Other classes of signals may be issued by the interrogator in response to the signal level received by the interrogator, or the history of signal levels received by the interrogator. A further class of signals may influence the behaviour of a label at any time in its reply cycle. A still further class of signals may influence the behaviour of a label only if it is received within a fixed portion of the reply cycle. Some of the interrogator signals may provide an indication of the length of time interval during which a reply from a label or labels has recently been received, and some signals may provide an indication of the uniformity of amplitude of the signal received by the interrogation during that period. The classes of transponder signals need not be distinct; a signal may belong to one or more classes.
According to the present invention there is provided a label reading system including:
an interrogator including a transmitter for generating an interrogation signal and a receiver for detecting and decoding a reply signal;
an interrogation field creation means including a transmitter antenna connected to said transmitter for generating from said interrogation signal an interrogation electromagnetic field through which an object possessing a code responding label may pass;
said code responding label including a label receiving antenna for receiving from said interrogation field a label interrogation signal, means for generating a label reply signal and means for generating from said label reply signal a reply electromagnetic field;
a receiver antenna connected to said receiver for receiving said reply signal from said label reply field; and
wherein said interrogator includes means for detecting strength of said reply signal and for generating a signal indicative of said strength.